Sangin's residents have criticised the planned withdrawal later this year of British troops from their town, complaining that four years of fighting have failed to bring peace or development.

"The British have failed," said Haji Fazlul Haq, a former town governor, speaking by telephone. "They could not bring security to the town and that is why they are handing it to the Americans."

The blunt assessment was shared by other residents who expressed greater confidence in US forces due to take control in November. "The Americans fight harder. I think the Taliban will be afraid of this change of command," said Haji Abdul Wahab, acting director of the peace commission of Helmand, a government body that promotes reconciliation.

Their reaction offered little consolation to British forces, who have paid a high price in Sangin. More than 100 British soldiers have died around the town since Tony Blair deployed the first troops in June 2006, making it the deadliest battlezone for western soldiers in Afghanistan.

British commanders say the withdrawal is not an admission of failure but rather a routine battlefield reorganisation that reflects the increased American presence in the province. There are currently 20,000 US soldiers in Helmand, twice as many as the British, and more are coming.

But there is little doubt that the British exit is an admission of the difficulty of purging the Taliban from a town in the grip of the heroin trade and surrounded by determined insurgents. While British soldiers have battlefield superiority over their enemy, many of whom are armed with basic rifles, the nature of the counter-insurgency requires them to conduct "presence patrols" in villages.

This makes them an easy target for insurgents who plant roadside bombs by night then melt into the population during the day. The Taliban are also becoming more canny: this year troops reported more landmines with no metal content, rendering metal detectors useless.

Some residents did recognise the British sacrifices. "I don't want to put all the blame on British because they have lost many lives here" said Shamsullah Khan, a candidate for September's parliamentary elections.

In Pakistan, the British move was seen as confirmation of the view that the Nato war in Afghanistan was unwinnable in its present form. "It's like chasing a nameless, invisible enemy," said Rustum Shah Mohmand, a former Pakistani ambassador to Afghanistan. "This is not about [Hamid] Karzai, or corruption, or the marginalisation of the Pashtuns. It's about the presence of the foreign forces. Until they leave, peace will never come to Afghanistan," he said.

It is not the only high-profile pullout in Afghanistan of recent months. In April, US forces withdrew from the Korengal valley in the east of the country, dubbed the "valley of death" by soldiers, ending years of intense but fruitless fighting. Forty-two Americans died and hundreds were wounded in fighting between 2006 and 2009 in the valley.

Nato has postponed a planned surge into Kandahar until the autumn amid fears of heavy Taliban resistance.

The operations are taking place against a backdrop of regional political intrigues. In recent months Karzai has held a series of meetings with Pakistan's army and intelligence chiefs, ostensibly with a view to negotiating a peace deal with the Taliban.

Riffat Hussain, a professor of defence studies at Quaid-e-Azam University in Islamabad, said the Sangin withdrawal was not likely to have a major impact on the Pakistan military's calculations.

"They're more concerned with what General [David] Petraeus is going to do. That's the major strategic concern right now," he said, referring to the US commander who took over from General Stanley McChrystal after comments about the Obama administration to Rolling Stone magazine cost McChrystal his job.

Petraeus has promised to review the rules of engagement governing Nato troops, which many soldiers had complained were too restrictive. But human rights activists and many Afghans worry that a change in the rules will lead to higher civilian casualties.

In Sangin, war-weary residents are bracing for an increase in violence as American troops try to succeed where the British failed. "One thing we know: the violence will increase," said Surat Khan, a brick-factory worker in Bastonzai, near Sangin. "The Americans will not be as defensive as the British. They are serious about conducting operations."

Solaiman Shah, a vegetable seller, was more sympathetic about the British losses. "The Brits have not helped us but they sacrificed a lot.

"It will take a long time for the Americans to build the same relationship with the people."